Hydraulic motors have been utilized to provide power to a negative mechanism (such as a motor for a drivewheel or winch) or to derive power from a positive mechanism (such as a fluid pump driven by a gasoline motor). In some instances, the device is also utilized for a secondary purpose such as controlling the speed of rotation of itself or an auxiliary member.
Most hydraulic devices are relatively large in diameter for a given volumetric efficiency. The reason for this is the constraints in the cross-sections of the fluid passages which are necessary in the body of such hydraulic device. Examples of devices with limited cross-sectional passages include the Ross Gear MF-MG series which include a separate series of set diameter holes interconnected in alternate plates by set diagonal passages to provide for a fluid path axially through the manifold between (and separately from) the main bolts. In this Ross device both the holes and lateral slots have limited cross-sections, thus limiting the amount of fluid which is able to pass axially through the manifold. Some devices partially neighbor a bolt—examples include the bi-directional valving passage in U.S. Pat. No. 5,173,043, Reduced Size Hydraulic Motor, and the uni-directional passages in U.S. Pat. No. 3,452,680, Hydraulic Motor Pump Assembly and U.S. Pat. No. 3,452,543, Hydrostatic Device. However this usage is limited to a single location surround (U.S. Pat. No. 5,173,043) or a symmetrical Passageway (U.S. Pat. Nos. 3,452,680, 3,452,543).